Bacteria are capable of reproducing at a very fast rate. Sometimes, a microorganism can survive exposure to antibiotic drugs. A misconception is that the bacteria somehow become "used to" the drug. However, just like all other living things, bacteria contain variations--genetic differences that may confer an advantage in a certain environment. In that case, if a bacteria has a random mutation that allows it to survive when antibiotics are present in their environment--a resistant gene, it can transfer this to other bacteria by conjugation. It also can reproduce rapidly by cell division, creating millions of other resistant bacteria. Once the drug wipes out all non-resistant strains, the resistant ones will persist as they are the "fittest". Ultimately, scientists continually need to create newer and stronger antiobiotics, as the old ones may become obsolete. This is showing Darwin's principles of Natural Selection, where the selecting agent is actually the antibiotic drug which applies a pressure on the bacteria population. Inappropriate use of antibiotics, like prescribing them for the common cold leads to antibiotic resistance. They are useless against the cold virus. Also, when people stop taking the full course of antibiotics, the weaker germs are killed, leaving behind the stronger, resistant ones. Resistant pathogens include, Staphlococcus aureus, which causes the dreaded infection MRSA in hospitals, Streptococcus pyogenes, which can cause necrotizing fascitis in which the patient has to have damaged tissue surgically removed and Streptococcus pneumoniae, which causes pneuominia, sinus, ear infections which are very difficult to treat. There are many additional resistant strains of bacteria, making their treatment more difficult for doctors.